1. Field of the Invention
The present invention relates to an automatic sheet feeder, and in particular, to an automatic sheet feeder for feeding stacked sheets one by one into an image forming section of a copying machine, a laser printer or the like.
2. Description of Prior Art
In recent years, sheet feeders of copying machines and printers have had increasing capacities in order to cope with copying of a large amount of sheets, and accordingly a variety of tandem stackers for mounting a sheet feed stacker with two stacks of sheets have been proposed. As shown in FIG. 4a, a prior art tandem stacker is made capable of moving upward and downward a first tray 101 positioned on the downstream side in a sheet feed direction P, and a stack of sheets S1 mounted on the first tray 101 are fed one by one from the uppermost one by a sheet feed roller 110. The first tray 101 moves upward according as the stack of sheets S1 reduces. On the other hand, a second tray 102 positioned on the upstream side in the sheet feed direction P is fixed in a lower position, and a stack of sheets S2 is mounted on the second tray 102. When the stack of sheets S1 is completely fed out, the first tray 101 moves downward, and a rod 103 moves in the sheet feed direction P, thereby sliding the stack of sheets S2 onto the first tray 101.
However, in the above-mentioned tandem stacker, lengths in the sheet feed direction P of the trays 101 and 102 correspond to the length of the sheets to be stacked on the trays 101 and 102. Therefore, when a vibration is applied to the sheet feeder, the stack of sheets S2 on the second tray 102 is displaced to the first tray 101 side. FIG. 4a shows a state in which the stack of sheets S2 is displaced by a length d. When the stack of sheets S2 is displaced as described above, a trouble occurs in moving upward and downward the first tray 101. Especially, as shown in FIG. 4b, the first tray 101 strands on the stack of sheets S2 when moving downward, possibly resulting in damaging the first tray 101 and its elevation mechanism.